Dissolution of potassium chloride ores



United States Patent Oifice 3,512,945 Patented May 19, 1970 3,512,945 DISSOLUTION OF POTASSIUM CHLORIDE ORES John Bryan Taylor, Ottawa, Ontario, Canada, and Michael Ralph Hunt, Surrey, England, assignors to Canadian Patents and Development Limited, Ottawa, Ontario, Canada, a corporation of Canada No Drawing. Filed Sept. 12, 1967, Ser. No. 667,072 Int. Cl. B01d 11/02; C01d 3/08 US. Cl. 23-312 4 Claims ABSTRACT OF THE DISCLOSURE When leaching or solution mining potassium salts including the chloride, manganese ion in at least about 100 p.p.m. is added to the leach solution to prevent obstructive sodium chloride recrystallization.

This invention is directed to the selective recovery of potassium salts from potassium chloride-containing mixtures or ores such as sylvinite by leaching or solution mining. The process of preferential leaching of potassium chloride with an aqueous sodium chloride solution is facilitated by adding manganese ion.

Sylvinite ore is primarily a physical mixture of sodium chloride and potassium chloride. It is present in the evaporite deposits of Stassfurt (Germany), Alsace, U.S.S.R., Carlsbad (New Mexico), Yorkshire (England) and Saskatchewan (Canada). It is a prime source of potassium chloride for use in fertilizers and to a lesser extent for use in the chemical industry. Other ores containing potassium chloride and sodium chloride may be treated according to the invention. Conventional procedures for mining involve dry shaft mining followed by a flotation or fractional crystallization process to separate the sodium and potassium chlorides. There is often little or no use for the sodium chloride obtained in this way because of the abundant supplies from halite beds. At depths greater than 3000 feet, e.g. southern part of the Saskatchewan deposit and the English deposit, dry mining is impracticable and solution mining becomes attractive. This process involves the sinking of bore holes and extraction of the ore body by circulating a suitable solvent.

Using water to extract sylvinite-type ore results in the complete removal of both sodium and potassium chlorides, but the sodium chloride has limited commercial value. It is therefore economically more desirable to attempt to extract only the potassium chloride. This can be done by using a leaching solution of saturated aqueous sodium chloride which is capable of dissolving potassium chloride but not sodium chloride. There is a major disadvantage associated with the use of this solution in that as potassium chloride is dissolved, some of the sodium chloride is rejected from the solution in the form of crystals (the effect is predictable from a knowledge of the phase equilibrium diagram of the system). Unfortunately, the recrystallized sodium chloride is deposited as a strongly adherent layer on the face of the dissolving ore and eventually the surface of the ore presented to the leach solution is composed entirely of sodium chloride. As a result the cavity within the ore bed becomes lined with sodium chloride, which is completely insoluble in the saturated sodium chloride leach solution and the process of dissolution stops.

It is possible to overcome the deposition of sodium chloride on the face of the dissolving ore by addition of trace quantities of certain metal ions to the saturated sodium chloride leach solution. These additives fall into a class generally referred to as crystal step poisons. In their presence, the recrystallization of sodium chloride occurs at a point remote from the actual ore face and also the crystals are modified in size and in some cases in shape (habit). As a result, an adherent deposit of sodium chloride does not form on the ore face.

US. Pat. No. 3,135,501 (June 2, 1964) covers the use of metal ions from Groups IV(A), V(A), VIII and II(B) of the periodic table but primarily this patent is concerned with addition of lead. However, our measurements of the effect of lead on the rates of dissolution of NaCl and KCl, and the known tendency of lead to be incorporated within the recrystallized sodium chloride indicates that its applicability to the solution mining operation would not be economically attractive.

An object of the present invention is to prevent detri mental recrystallization of sodium chloride without at the same time decreasing the rate of dissolution of potassium chloride. A further object is to provide an additive for the leach solution which minimizes adherent sodium chloride recrystallization and is not depleted from the leach solution by inclusion in the recrystallizing sodium chloride. Another object is to provide a leach solution which is effective in the preferential leaching of potassium chloride.

According to the present invention manganese ion is added to the aqueous sodium chloride leach solution in amounts of at least about 100 p.p.m. by weight of the water. The preferred concentration of manganese ion is about 500 to about 1000 p.p.m. and the manganese ion is preferably added as manganous chloride. The manganese may be added as a manganous salt of any anion which does not interfere with the dissolution process. Manganese minimizes or prevents the adherent recrystallization of sodium chloride on the ore face when a leach solution of aqueous sodium chloride is employed. Unlike lead, manganese does not retard the rate of dissolution significantly and has previously been shown to not be depleted from the solution by incorporation within recrystallizing sodium chloride.

The leach solution may be saturated aqueous sodium chloride, or unsaturated NaCl solutions may be used particularly when experience with a given ore indicates advantage therewith. Unsaturated leach solutions would not usually contain less than about of the saturation amount of sodium chloride.

The following examples illustrate the invention:

EXAMPLE 1 Slices of ore 5 inches wide by 6 inches high by /2 inch thick sandwiched and cemented between transparent plastic plates (Lucite-trademark) were suspended vertically in a tank of leach solution (36 litres) at 36 C. Progress of dissolution was followed by time lapse motion picture photography. Two grades of sylvinite ore were studied containing 60% KCl and 23% KCl. The former represents the middle grade ores which were shown by autoradiography using the K activity to have an essentially contiguous association of KCl crystals, while the latter represent the lower grade ores having isolated KCl crystals in a sodium chloride matrix. Aqueous leaching solutions used were saturated sodium chloride and 94% saturated sodium chloride.

The dissolution of individual crystals of KCl in the leach. solutions were also observed under a microscope.

With the saturated solution, as the potassium chloride dissolved some of the sodium chloride in solution was observed to deposit on the KCl face as crystals which eventually formed a strongly adherent deposit on the ore face and stopped the KCl dissolution. Concentrations of lead ion in the range to 500 p.p.m. by weight added to the saturated NaCl solution had a marked elfect on the nature of the recrystallized NaCl (the crystals were not interconnected and fell freely away from the dissolved face). Below 100 p.p.m. the effect of lead decreased. The addition of 500 p.p.m. Mn++ by weight (added as MnCl .4H O) had an effect similar to that of 100 p.p.m. lead (an adherent layer of NaCl did not develop on the ore face). The addition of Mn++ion in amounts as low as about 100 p.p.m. was observed to have significant beneficial effect, while increasing the concentration from 500 to 1000 p.p.m. did not significantly enhance the effect. No critical upper concentration limit exists (except saturation).

The low grade (23% KCl) ore was found to not dissolve in the saturated leach solution and to give unsatisfactory preferential leaching of the KCl in unsaturated leach solutions. All of the NaCl was removed as well as the KCl and while recrystallization of the NaCl on the KCl could be prevented, no benefit was obtained by using a leach solution containing NaCl with low grade ores. Preliminary studies indicate that benefits of the invention will be realized with ores having a substantially continuous KCl phase. This condition would in general be fulfilled in ores containing greater than about 40% KCl (although there is no direct relationship between KCl concentration and KCl distribution).

The unsaturated leach solutions resulted in the dissolution proceeding more rapidly than with saturated solutions. Due to localized saturation conditions adherent recrystallization of NaCl occurred, but was prevented by the addition of manganese. In the middle and rich grades of ore not all of the NaCl was dissolved, particularly with saturated leach solutions, and some NaCl crystals (or KCl poor ore fragments) were observed to become detached and fall away from the dissolving face.

EXAMPLE 2 Because of the lack of availability of exactly duplicate ore samples and also because of uncontrollable variables such as impurities carried into the solution from the ore itself, it was decided to make kinetic measurements on single crystals of NaCl and KCl. Rates of dissolution of. the cube faces were measured for NaCl dissolving in 95% saturated aqueous NaCl, and KCl dissolving 95 saturated aqueous KCl. To the leach solution or PbClwas added as required to give the Mn++ and Pb++ concentrations indicated.

NaCl dissolving in 95 NaCl Concentration of Mn++ Concentration of Pb++ in solution (p.p.m.)

Rate of dissolution of 4 KCl dissolving in KC] Rate of dissolution of cube face (relative) Concentration of Mn++ in solution (p.p.m.)

It will be observed that in the presence of lead ion the rates of dissolution decreased significantly whereas in the presence of Mn ion the rates were only slightly reduced. An additional advantage of Mn++ over Pb++ is that if slightly undersaturated solutions of NaCl are used as leach with the intention of removing some NaCl from the ore (perhaps with the aim of exposing KCl more rapidly), then Mn++ will have little effect on the kinetics of this process whereas Pb++ will severely retard the dissolution of NaCl.

We claim:

1. A method of preventing adherent sodium chloride recrystallization from an aqueous leach solution onto a dissolving surface while dissolving potassium salts from mixtures and ores containing sodium and potassium chloride, comprising adding a least about p.p.m. of manganous ion to said leach solution at least about 90% weight saturated in sodium chloride extracting the mixture or ore with said aqueous leach solution, and recovering the potassium from the resulting leach solution.

2. The method of claim 1 wherein about 100 to about 1000 p.p.m. manganous ion is present in the leach solution.

3. The method of claim 1 wherein sylvinite ore containing at least about 40% potassium chloride is leached.

4. The method of claim 1 wherein substantially saturated sodium chloride solution is used as leach.

References Cited UNITED STATES PATENTS 2,211,397 8/1940 Weining 23-312 X 3,135,501 6/1964 Dohms 299-5 X 3,215,471 11/1965 Gunning 23-312 X 3,341,453 9/1967 Ralston 23-293 X 3,378,489 4/1968 Lasater 23303 X 3,441,386 4/1969 Veronica 23312 X FOREIGN PATENTS 482,579 3/ 1938 Great Britain.

NORMAN YUDKOFF, Primary Examiner S. J. EMERY, Assistant Examiner U.S. Cl. X.=R. 2389, 293, 300; 299-5 

